1. Field of the Invention
The invention is related to two channel fiber optic rotary joint in the field of optic transmission through a mechanical rotational interface.
2. Description of Related Art
The Fiber optic Rotary Joint (FORJ) is the opto-mechanical device which allows uninterrupted transmission of an optical signal in a fiber guide through a rotational interface to a stationary apparatus. FORJ can be categorized as active and passive. An active FORJ consists of a light on either rotor side or stator side and a photo detector on another side and the signal transmission is limited to unidirectional. The disadvantage of active FORJ is requirement for electrical power. The passive FORJ is intended to transfer optical signals from fiber to fiber without any electronic or electrical units. The use of FORJ can be widely found in missile guidance systems, robotic systems, remotely operated vehicles (ROVs), oil drilling systems, sensing systems, and many other field applications where a twist-free fiber cable is essential. Combined with electrical slip rings or fluid rotary joints, FORJs add a new dimension to traditional rotary joints. As fiber optic technology advances, more and more traditional slip ring users will benefit from FORJs in their new fiber systems. This issue can be solved relatively easy if only a single channel is to be transmitted because it can be transmitted by keeping alignment between the optical axis and mechanical rotational axis. However, the transmission results in difficulties when it is desired to transmit two channels separately from each other through a single rotation interface.
A couple of prior inventions of two channel fiber optic rotary joint are described in the following patents: U.S. Pat. No. 5,588,077, U.S. Pat. No. 4,842,355, and U.S. Pat. No. 47,251,116.
In U.S. Pat. No. 5,588,077, the two optic fiber channels are arranged in-line along the same rotational axis. Isolation of one channel from the other is achieved through a novel application of gradient index rod lenses of suitable pitch. A pair of lenses is arranged adjacent each other on each side of the rotational interface and a second pair of axially aligned lenses is arranged outboard of the first pair. An optic signal from one of the outboard lenses can be directed to one of the other lenses depending on the pitch selection. The drawback of this design is that the losses due to crosstalk and overlap of the signal paths would be pretty significant.
Gold, et al designed another two channel FORJ in U.S. Pat. No. 4,842,355. A first channel signal is delivered to an optic fiber transmitted coaxially of the stationary and rotary side, transfer across the rotational plane between the two components being accomplished by opposing centrally located optic lenses. A second channel transmitted through a second optic fiber is delivered to a lens system which converts the light into a cylinder of light coaxial with the first channel and which surrounds the optic management for the first channel. Second channel thus are converted into coaxial hollow cylinders of light. These cylinders of light are transmitted between facing lens systems in the rotary and stationary sides of the apparatus. But the facing lens systems are very difficult to be fabricated.
Spencer, et al shows in U.S. Pat. No. 47,251,116 a two-channel and multi-channel FORJ. Within the joint reflecting surface are used to redirect off-axis optic signals onto the joint axis, with relative rotation occurring while the signals are on-axis. A rotating member for each channel has a reflecting surface for reflecting the on-axis signal portion off-axis to a receptor fiber. Alignment between the rotating member and the receptor fiber, as well as drive for the rotating member, is provided by a pair of magnets of opposite polarity, one being secured to the rotating member and the other being secured to the rotor. But it could be very difficult for the magnetic interaction to accurately ensure the synchronous rotation of the rotor and the rotating member. The size of the magnetic element and the adjustment of the reflecting surface also increase the size of the invented embodiment.